The present invention relates to universal exhaust gas oxygen sensor control circuits providing a universal exhaust gas oxygen (UEGO) sensor interface and an output for an engine control module and employing an application specific integrated circuit (ASIC) with a plurality of control circuits.
Since fuel economy and improved power are desirable in the operation of internal combustion engines such as those used in automobiles, exhaust gas oxygen sensors have been used to indicate whether the air-fuel mixture is lean or rich for the current performance of the engine. While some sensors provide merely positive or no output in response to whether the mixture is rich or lean, universal exhaust gas oxygen (UEGO) sensors have been developed to provide multiple indications of the degree of adjustments needed to conform to a desired air-fuel ratio.
In general, a UEGO sensor includes a chamber with a controlled oxygen content separated by a permeable membrane from an adjacent chamber that receives exhaust gas oxygen. A reference voltage associated with the referenced oxygen content varies as the referenced oxygen content changes in response to the amount of oxygen contained in the adjacent chamber. A sensor control circuit for the UEGO sensor generates a pumping current and attempts to readjust the level of oxygen contained in the reference chamber to balance towards the optimum air fuel mixture. As a result, the pumping current reflects the richness or leanness of the air-fuel mixture being fed for combustion. The pumping currents may be monitored for adjustment of the air-fuel ratio in the engine control module. The sensor may be heated by a heater control circuit to maintain ambient conditions compatible with exhaust gas sensing.
The previously known UEGO sensors employ individual control circuits. A stand-alone unit such as an NTK Corporation T-6000 stand-alone UEGO controller controls a single sensor. A multiple sensor control system would require installation of multiple control circuits. As a result, a substantial amount of circuitry is required to be packaged near or on the engine with the engine controls. Moreover, the UEGO sensors operate in the harsh environment of exhaust gases, and thus subject locally located circuitry to heat, temperature variation, and vibration problems. In addition, in systems where multiple sensors are to be employed in order to properly adjust air fuel ratio for the engine, the exposure of multiple circuits and the difficulty of packaging the circuits in the automobile substantially reduce the reliability of such systems as previously employed.
The present invention overcomes the above-mentioned disadvantages by providing UEGO sensor control circuitry employed in large part on an application specific integrated circuit (ASIC) which, together with other associated circuitry, is particularly well adapted to communicate with or form a part of the engine control module. In general, the ASIC is constructed to perform sensor control functions including proportional-integral-derivative control processes for evaluating the air-fuel ratio and its relationship to the desired air-fuel ratio. In addition, sensing cell drivers, the heater control for the sensor, SPI communication control for compatible processing of information with the engine control module, a common power supply voltage generator (V com) and UEGO output buffers are formed wholly on the ASIC. Additional circuitry, such as a trim compensation circuit that compensates for sensor deviations from ideal manufacturing specifications, sensor interface circuitry that filters and conditions sensor signal outputs for input to the ASIC, and heater drive monitoring circuitry, all of which may be dependent upon the type of sensors employed, are preferably formed on a circuit board portion that is preferably a part of a circuit board in the engine control module. An output circuit that conditions the control circuit operation as input to the engine control module may also be populated on the circuit board portion.
In a preferred embodiment, a plurality of UEGO sensors are heated by heater circuitry and operated by controls in the ASIC. Sensors such as the NTK TL-7111-W1 include a trim resistor designating the type of variation exhibited by the sensor from ideal specifications that occurs during manufacturing. The compensation circuit forms a voltage-clamped, voltage divider providing a compensation signal to an analog to digital (A-D) converter at the engine control module. In the preferred embodiment, output circuitry that receives replicated sensor pumping current transforms the current into a differential voltage output that is then converted to a single-end or voltage output that is filtered for receipt at the engine control module. The output circuitry preferably includes voltage clamp for avoiding excessive input to the engine control module. Moreover, all of the above engine circuits may be provided on a portion of a circuit board that forms a part of a circuit board in the engine control module.
As a result, the present invention provides a UEGO sensor control circuit that has a substantially smaller package than previously known UEGO sensor control circuits. Moreover, the reliability of the circuits is substantially improved, and the circuit becomes more robust in the harsh environment of the engine compartment.